The present invention relates to a method and an apparatus for the orientation of a plastic pipe, wherein a pipe blank is produced with an extruder and fed into a pipe forming apparatus provided with movable moulds, such as a corrugator, the pipe being conveyed forward in the corrugator with said moulds over a mandrel, along which the pipe is further conveyed out from the corrugator, and an oriented ribbed plastic pipe produced by the method.
The orientation of plastic articles means enhancing the strength of said articles in a certain direction by orienting the molecules in the plastic material in said direction, whereby the tensile strength of the plastic increases and the stretch decreases in said direction. Applied to tubular articles, orienting is effected in the radial direction (radial orientation), whereby for instance the pressure resistance of the pipe increases, or in the longitudinal direction of the pipe (axial orientation), whereby for instance the tensile strength of the pipe increases, or in both directions (biaxial orientation).
The production of unoriented corrugated plastic pipes or ribbed pipes is well known technology, cf. for instance Finnish Patents 60825, 74654 and 77405. Improving the properties of ribbed pipes by orientation is also known, cf. for instance the publication WO 90/02648. The orientation technique of normal plastic pipes has been disclosed for instance in the publication DE 2357210 (Petzetakis).
In the known methods and apparatuses for producing plastic pipe wherein a corrugator is used the molten tubular blank extruded from the die is moulded and cooled by means of movable moulds (cast moulds). Since the plastic material must be made to set rapidly, an interior mandrel is often used to aid the cooling. With a moving chain of moulds, highly efficient heat transfer is achieved. Typically, in the production PVC pipes, for instance, the temperature of the material is 200.degree. C. at the feeding step, and is cooled by means of moulds to about 50.degree. C. In the radial orientation of conventional pipes, the material is forced onto a conically widening second mandrel, and thus the wall of the tube is stretched and the plastic molecules are oriented in the radial direction and possibly also axially (biaxial orientation). Depending on the conditions, biaxial orientation can also be effected in different directions separately in two or more steps.
The known orientation techniques are characterized in that the pipe is cooled on the outside with a purpose of providing for the pipe as rapid cooling down to the orientation temperature as possible. Thereafter the pipe is conveyed for instance with a drawing apparatus further to the next processing step, for instance to an orientation station, at which the orientation is performed. The orientation temperature is advantageously about 10.degree.-30.degree. C. higher than the glass transition temperature of the plastic. What is important is that the temperature gradient in the pipe wall is uniform.
When a pipe in this state is oriented by drawing it over a widening mandrel portion, high friction forces are generated between the mandrel and the plastic pipe. On the other hand, in the known apparatuses the soft pipe would not withstand drawing over the mandrel without lubrication (cf. DE 2357210). In the known plastic pipe manufacturing lines, the cooling and transport of the pipes is carried out with separate equipment, in which situation considerable attention must be paid to the heat treatment and mechanical strength of the pipe.